Tapping screw

ABSTRACT

The tapping screw has a screw thread having a non-circular shape. Accordingly, when the tapping screw is screwed into the prepared hole formed in the attached member such as a steel plate, etc., the contact area of the screw thread and the circumferential face of the prepared hole of the attached member can be reduced in comparison with the screw thread having a circular outer shape. Accordingly, the screwing torque can be reduced. Further, since the top portion of the screw thread of the square outer shape has a rising shape without being broken, the biting property of the screw thread with respect to the prepared hole formed in the attached member is improved so that a female screw can be precisely threaded on the inner circumferential face of the prepared hole by the screw thread.

TECHNICAL FIELD

The present invention relates to a tapping screw, and particularly,relates to a tapping screw in which the outer shape of a screw threadseen from the axial direction is a non-circular shape.

BACKGROUND ART

A conventionally proposed tapping screw is screwed into a prepared holeformed in an attached member so that the tapping screw is fastened tothis prepared hole while the tapping screw itself threads the screw.When the screw is threaded in the prepared hole by the tapping screw,the screw threading is advanced by hitting the top portion of the screwthread against the circumference face of the prepared hole. Here, it ispublicly known that one portion of a shaft portion is formed in atapering off shape, or the outer shape of the screw thread seen from itsaxial direction is a non-circular shape to more efficiently form thescrew in the prepared hole. For example, various outer shapes such as atriangular shape (Japanese rice ball shape) having round corners, or anelliptical shape, etc. are proposed as the outer shape of the screwthread seen from its axial direction (e.g., see Patent Document 1).

[Patent Document 1]

JP-A-11-247817 (paragraphs 8 to 9, FIG. 1)

DISCLOSURE OF THE INVENTION

However, in the above prior art, for example, when the screw threadforming the screw in the prepared hole is broken in fastening thetapping screw to a high tensile steel plate, etc., a problem exists inthat the tapping screw cannot be fastened to the partner steel plate.Further, when the screw thread of the non-circular outer shape is notcompletely pointed in the taper portion, it is difficult to preciselythread the screw in the prepared hole. As a result, a problem exists inthat large thrust and screwing torque are required when the portion ofthe screw thread that is not pointed is screwed and fastened to theprepared hole.

An object of the present invention is to provide a tapping screw inwhich the screwing torque is reduced and the tapping screw is easilyscrewed into the prepared hole of the attached member.

In the invention of claim 1, plural notch portions are formed in aspecific area of a screw thread continuously formed over a constantdiameter portion and a reduced diameter portion. The notch portion has anotch face, a rising wall and a corner portion. The biting property ofthe screw thread with respect to a prepared hole of an attached memberis improved by forming the notch portion. Therefore, a female screw isreliably formed in the prepared hole of the attached member. Further, inthe notch portion, the corner portion mainly comes in contact with thecircumferential face of the prepared hole. Thus, the contact area of thescrew thread bitten into the prepared hole of the attached member isreduced. Therefore, screwing torque of the tapping screw into theprepared hole can be reduced. Accordingly, the screwing torque isreduced and the tapping screw can be easily screwed into the preparedhole of the attached member. Further, the plural notch portions areformed in the circumferential direction of the screw thread. Therefore,the female screw can be precisely formed on the circumferential face ofthe prepared hole.

In the invention of claim 2, the notch face has a stem face notched andformed on the diametrical inside from the top of the screw thread.Therefore, in the screw thread, the corner portion mainly comes incontact with the circumferential face of the prepared hole in the notchportion. Thus, the contact area of the screw thread bitten into theprepared hole of the attached member is reduced. Accordingly, thescrewing torque can be reduced.

In the invention of claim 3, the rising wall is approximatelyperpendicular to the screw rotating direction. Therefore, the bitingproperty of the screw thread with respect to the prepared hole of theattached member is improved. Thus, the female screw is reliably formedin the prepared hole of the attached member. Accordingly, the screwingtorque is reduced and the tapping screw can be easily screwed into theprepared hole of the attached member.

In the invention of claim 4, the corner portion is formed at thediametrical outside end of the rising wall. The corner portion pushesand widens the prepared hole by biting onto the circumferential face ofthe prepared hole. The screw thread and the notch portion are molded byform rolling. Thus, the shape accuracy of the notch portion is improvedand the corner portion is formed in a clear and sharp shape. Therefore,the biting property of the screw thread with respect to the preparedhole of the attached member is improved. Thus, the female screw isreliably formed in the prepared hole of the attached member. Further, inthe screw thread, the corner portion mainly comes in contact with thecircumferential face of the prepared hole in the notch portion.Therefore, the contact area of the screw thread bitten into the preparedhole of the attached member is reduced. Accordingly, the screwing torqueis reduced and the tapping screw can be easily screwed into the preparedhole of the attached member. Further, the screw thread is easily moldedby molding the notch portion by form rolling. Accordingly, productivitycan be improved.

In the invention of claim 5, the stem portion is formed from a positionmoved a predetermined angle a from the corner portion on the backwardside in the rotating direction to the end portion of the rising wall onits diametrical inside. Therefore, the contact area of the screw threadwith respect to the prepared hole of the attached member is reduced.Accordingly, the screwing torque is reduced and the tapping screw can beeasily screwed into the prepared hole of the attached member.

In the invention of claim 6, 7 or 8, the screw thread is left in thespecific area. Namely, the notch portion is formed by notching oneportion of the screw thread. Therefore, in comparison with a case inwhich the screw thread is deeply notched, the screw thread and the notchportion are easily formed, and the life of a form rolling die isextended. Further, since the notch portion is formed by the formrolling, the corner portion is formed in a clear and sharp shape. As aresult, the biting property of the screw thread with respect to theprepared hole of the attached member is improved. Thus, the female screwis reliably formed in the prepared hole of the attached member.Accordingly, the screwing torque is reduced and the tapping screw can beeasily screwed into the prepared hole of the attached member. Further,productivity can be improved since the screw thread and the notchportion are easily formed and the life of the form rolling die isextended.

In the invention of claim 9, the outer shape seen from the axialdirection of the screw thread is set to be non-circular in the specificarea of the screw thread continuously formed over the constant diameterportion and the reduced diameter portion. Further, with respect to thescrew thread having the non-circular outer shape, the contact area withthe circumferential face of the prepared hole of the attached member isreduced in the specific area. Accordingly, the screwing torque isreduced and the tapping screw can be easily screwed into the preparedhole of the attached member.

In the invention of claim 10, the outer shape of the constant diameterportion seen from the axial direction of the screw thread is set to benon-circular in a predetermined range in the axial direction from theboundary of the constant diameter portion and the reduced diameterportion. Thus, the contact area of the screw thread and thecircumferential face of the prepared hole of the attached member isreduced in one portion of the constant diameter portion and the reduceddiameter portion. Accordingly, the screwing torque is reduced and thetapping screw can be easily screwed into the prepared hole of theattached member. Further, after the reduced diameter portion is screwedinto the prepared hole, the female screw of the same diameter as theconstant diameter portion is threaded in the prepared hole in advancebefore the constant diameter portion is fastened. Accordingly, theconstant diameter portion can be easily fastened.

In the invention of claim 11, the shape of the screw thread seen fromits axial direction in the specific area is a square shape. Therefore,the top portion of the square screw thread is formed in a rising shapewithout being broken. Thus, the biting property of the screw thread withrespect to the prepared hole of the attached member is improved.Accordingly, the female screw can be reliably formed in the preparedhole.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view showing a tapping screw inaccordance with a first embodiment mode of the present invention.

FIG. 2 is a plan view showing the tapping screw in accordance with thefirst embodiment mode of the present invention.

FIG. 3 is a bottom view showing the tapping screw in accordance with thefirst embodiment mode of the present invention.

FIG. 4 is a cross-sectional view cut along line A-A in FIG. 1.

FIG. 5 is a schematic perspective view showing a tapping screw inaccordance with a second embodiment mode of the present invention.

FIG. 6 is a bottom view showing the tapping screw in accordance with thesecond embodiment mode of the present invention.

FIG. 7 is a bottom view typically showing the outer shape of a screwthread of the tapping screw in accordance with the second embodimentmode of the present invention.

FIG. 8 is a perspective view typically showing one portion of the screwthread of the tapping screw in accordance with the second embodimentmode of the present invention.

FIG. 9 is a typical view of the outer shape of the screw thread of thetapping screw in accordance with the second embodiment mode of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Plural embodiment modes applying the present invention thereto will nextbe explained on the basis of the drawings.

First Embodiment Mode

A first embodiment mode of a tapping screw embodying the presentinvention will be explained with reference to FIGS. 1 to 4. Here, thetapping screw of this embodiment mode is screwed into a prepared holeformed in an unillustrated attached member such as a steel plate, etc.so that the tapping screw is fastened to the attached member while afemale screw is formed in that prepared hole. As shown in FIGS. 1 and 2,the tapping screw 1 is constructed by a head portion 2 of a hexagonalcolumnar shape, and a shaft portion 3 extending downward from the lowerside face 2 b of this head portion 2. In the tapping screw 1, both thehead portion 2 and the shaft portion 3 extend along the common axis L. Across hole 15 is formed on the upper face 2 a of the head portion 2, andthe tip of an unillustrated tool for transmitting torque to the tappingscrew 1 is fitted into the cross hole 15. The shaft portion 3 isconstructed by a constant diameter portion 3 a approximately formed in acolumnar shape and extending from the head portion 2, and a reduceddiameter portion 3 b approximately formed in a truncated cone shape andextending from the tip of the constant diameter portion 3 a while beinggradually reduced in diameter toward the direction opposed to the headportion 2.

The constant diameter portion 3 a is formed in the columnar shape havinga constant diameter from its base end portion to the tip portion. Thereduced diameter portion 3 b has a base end portion of the same diameteras the tip portion of the constant diameter portion 3 a, and is formedin the truncated cone shape gradually reduced in diameter from the baseend portion to the tip portion. The reduced diameter portion 3 b isreduced in diameter at a taper angle of about 10° with respect to theconstant diameter portion 3 a. The taper angle can be arbitrarilychanged in accordance with the applied attached member. In the rangefrom the vicinity of the base end of the constant diameter portion 3 ato the tip of the reduced diameter portion 3 b, a screw thread 11 of apredetermined pitch is continuously formed in its outer circumferentialportion. An unthreaded lower neck portion 18 may be also formed in thebase end portion of the constant diameter portion 3 a. The screw thread11 is formed in a spiral shape in the outer circumferential portion ofthe shaft portion 3 except for the lower neck portion 18.

As shown in FIG. 3, when the screw thread 11 is seen from the bottomface side in the axial direction, the outer shape of the screw thread 11in the tapping screw 1 is approximately a square shape having roundcorners from the tip portion of the reduced diameter portion 3 b throughthe base end portion in the range from this base end portion to oneportion of the tip side of the constant diameter portion 3 a. As shownin FIG. 1, the outer shape of the screw thread 11 is a square shapehaving round corners from the tip of the reduced diameter portion 3 b inthe head portion 2 direction via the tip portion of the constantdiameter portion 3 a in a range of two pitches of the screw thread 11from this tip portion in the base end portion direction. The outer shapeof a root portion of the screw is also formed in a square shape havinground corners correspondingly to the screw thread 11 although thissquare shape is unillustrated. Further, as shown in FIG. 4, in a sectionin which the tapping screw 1 is cut in an arbitrary plane including itsaxis L, the top portion of the screw thread 11 is formed in a risingshape not broken in all forming ranges thereof including an area inwhich the outer shape of the screw thread 11 seen from its axialdirection is approximately formed in the square shape.

As explained above, the tapping screw 1 of the first embodiment mode hasthe screw thread 11 having the square outer shape. Therefore, when thetapping screw 1 is screwed into the prepared hole formed in theunillustrated attached member such as a steel plate, etc., the contactarea of the screw thread 11 and the circumferential face of the preparedhole of the attached member is reduced in comparison with theconventional tapping screw, i.e., the screw thread having the circularouter shape. Accordingly, the screwing work can be made by screwingtorque smaller than that in the conventional case. Further, the topportion of the screw thread 11 having the square outer shape is notbroken, but is formed in the rising shape. Therefore, the bitingproperty of the screw thread 11 with respect to the prepared hole formedin the attached member is improved. Accordingly, a female screw can beprecisely threaded on the inner circumferential face of the preparedhole by the screw thread 11.

Second Embodiment Mode

A second embodiment mode of the tapping screw embodying the presentinvention will next be explained with reference to FIGS. 5 to 9. Here,the tapping screw of this embodiment mode is screwed into a preparedhole formed in an unillustrated attached member such as a steel plate,etc. so that the tapping screw is fastened to the attached member whilea female screw is formed in that prepared hole.

First, the entire structure of the tapping screw 30 will be explainedwith reference to FIG. 5. As shown in FIG. 5, the tapping screw 30 isconstructed by a head portion 32 of a hexagonal columnar shape, and ashaft portion 33 extending downward from the lower side face 32 a ofthis head portion 32. In the tapping screw 30, both the head portion 32and the shaft portion 33 extend along the common axis M. Anunillustrated cross hole is formed on the upper face of the head portion32, and the tip of a tool for transmitting torque to the tapping screw30 is fitted into this cross hole.

The shaft portion 33 is constructed by a constant diameter portion 33 aapproximately formed in a columnar shape and extending from the headportion 32, and a reduced diameter portion 33 b approximately formed ina truncated cone shape and extending from the tip of the constantdiameter portion 33a while being gradually reduced in diameter towardthe direction opposed to the head portion 32. The constant diameterportion 33 a is formed in the columnar shape having a constant diameterfrom its base end portion to the tip portion. The reduced diameterportion 33 b has a base end portion of the same diameter as the tipportion of the constant diameter portion 33 a, and is formed in thetruncated cone shape gradually reduced in diameter from the base endportion to the tip portion. The reduced diameter portion 33 b is reducedin diameter at a taper angle of about 10° with respect to the constantdiameter portion 33 a.

As shown in FIGS. 5 and 6, in the range from the vicinity of the baseend portion of the constant diameter portion 33 a to the tip portion ofthe reduced diameter portion 33 b, a screw thread 40 of a predeterminedpitch is continuously formed in its outer circumferential portion. Anunthreaded lower neck portion 19 may be also formed in the base endportion of the constant diameter portion 33 a. The screw thread 40 isformed in a spiral shape in the outer circumferential portion of theshaft portion 33 except for the lower neck portion 19.

In the tapping screw 30, a notch portion 45 is formed in the top portion41 of the screw thread 40 from the tip of the reduced diameter portion33 b in the head portion 32 direction via the tip portion of theconstant diameter portion 33 a in a range of two pitches of the screwthread 40 from this tip portion to the base end portion side. As shownin FIG. 6, when the screw thread 40 is seen from the bottom face side inthe axial direction, plural notch portions 45 are formed in the topportion 41 of the screw thread 40 in approximately equal positions withrespect to the circumferential direction of the shaft portion 33. In thecase of the second embodiment mode, the notch portions 45 are equallyformed in four places in the circumferential direction. All the notchportions 45 are formed in the same shape. The notch portions 45 areformed every four notch portions with respect to the screw thread 40 ofone pitch so as to be overlapped with each other when they are seen fromthe bottom face. Namely, in the top portion 41 of the screw thread 40 ofone pitch, the four notch portions 45 are formed one by one at theinterval of a rotating angle of 90° in the circumferential direction ofthe shaft portion 33 when they are seen from the bottom face.

Next, the outer shape of the screw thread 40 forming the notch portion45 in the top portion will be explained in detail with reference to FIG.7. Here, FIG. 7 is a bottom view typically showing the outer shape ofthe screw thread 40. As shown in FIG. 7, when the screw thread 40 formedin the shaft portion 33 is seen from the bottom face side in the axialdirection, the round circle of the screw thread 40 is divided into fourequal fan-shaped areas, and the circumferential edge portion of eacharea is formed in a shape notched in a semi-crescent shape by the notchportion 45. Thus, the screw thread 40 is approximately formed in awindmill type shape as a whole.

The notch portion 45 has a rising wall 60, a corner portion 65 and anotch face 66. The rising wall 60 is approximately perpendicular to therotating direction of the tapping screw 30. The rising wall 60 isconnected to each of the adjacent notch faces 66 inside and outside thetapping screw 30 in its diametrical direction. Namely, the rising wall60 connects two notch faces 66 adjacent to each other in thecircumferential direction to each other in the circumferential directionof the tapping screw 30. Thus, the end portion of the notch face 66 onthe forward side in the rotating direction of the tapping screw 30 isconnected to the end portion of the rising wall 60 on its outside in thediametrical direction, and the end portion of the notch face 66 on thebackward side is connected to the end portion of the adjacent risingwall 60 on its inside in the diametrical direction.

The corner portion 65 is formed in the connecting portion of the notchface 66 and the end portion of the rising wall 60 on the outside of thetapping screw 30 in the diametrical direction. The corner portion 65comes in contact with the prepared hole formed in the attached member asthe tapping screw 30 is rotated. Thus, the corner portion 65 pushes andwidens the inner wall of the prepared hole and forms a female screw.

The notch face 66 connects the adjacent rising walls 60 to each other asmentioned above. The notch face 66 is an end face of the screw thread 40on its outside in the diametrical direction formed in the tapping screw30. Namely, in the tapping screw 30, the screw thread 40 is left even inthe forming area of the notch portion 45. In other words, the notchportion 45 is in a shaving state of the diametrical outside of the screwthread 40, i.e., its top. One end portion of the notch face 66 isconnected to the root of the rising wall 60 in the circumferentialdirection of the tapping screw 30, and the other end portion isconnected to the tip of the rising wall 60, i.e., the corner portion 65.On the notch face 66, the axial length of the tapping screw 30 isgradually reduced from one end portion side connected to the root of therising wall 60 to the other end portion side connected to the cornerportion 65 in the circumferential direction of the tapping screw 30 asshown in FIG. 8. This is because the axial length of the screw thread 40is reduced from the inside to the outside in the diametrical direction.Namely, this is because the axial length of the screw thread 40 is longinside the rising wall 60 in the diametrical direction, but is shortoutside the rising wall 60 in the diametrical direction.

As shown in FIG. 9, the notch face 66 has a stem face 67 in one portionof the tapping screw 30 in its circumferential direction. FIG. 9 is atypical view showing a sectional shape of the tapping screw 30 andshowing one pitch of the screw thread 40.

The notch face 66 has a circumferential face 68 having a constantoutside diameter in the range of a predetermined central angle a fromthe corner portion 65 in the circumferential direction of the tappingscrew 30. In contrast, the outside diameter of the notch face 66 isgradually reduced and the notch face 66 enters the central side from theend portion of the circumferential face 68 on its side opposed to thecorner portion to the root of the rising wall 60. A portion changed inthe outside diameter among this notch face 66 is the stem face 67.Namely, one end portion of the stem face 67 in the circumferentialdirection of the tapping screw 30 is connected to the end portion of thecircumferential face 68 on its side opposed to the corner portion, andthe other end portion is connected to the end portion of the rising wall60 on its diametrical inside. In the case of this embodiment mode, thecentral angle α is about 20°. In addition, for example the central angleα can be arbitrarily set in accordance with the material of the attachedmember applying the tapping screw 30 thereto, the inside diameter of theprepared hole, etc.

As mentioned above, in the tapping screw 30, the screw thread 4 0 isleft even in the forming area of the notch portion 45. Namely, as shownin FIG. 8, the screw thread 40 has a predetermined height from the screwbottom portion 42 to the root of the rising wall 60 even in theconnecting portion of the notch face 66 lowest in the screw thread 40and the root of the rising wall 60.

As explained above, in the tapping screw 30 of the second embodimentmode, the notch portions 45 equally located in the circumferentialdirection of the shaft portion 33 are formed in the screw thread 40surrounding the shaft portion 33 in a spiral shape in a predeterminedrange along the axial direction from the tip of the shaft portion 33. Inthe notch portion 45, the corner portion 65 located in a circumferentialedge portion is located on the diametrical outermost side of the screwthread 40, and is molded together with the screw thread 40 by formrolling. Thus, the corner portion 65 is formed in a clear and sharpshape. Accordingly, when the tapping screw 30 is screwed into theprepared hole formed in the attached member, the corner portion 65 canprecisely thread the inner wall of the prepared hole. Further, therising wall 60 of the notch portion 45 is perpendicular to the rotatingdirection of the tapping screw 30, and its surface is in a state inwhich this surface is always directed forward in the rotating direction.Thus, as the tapping screw 30 is rotated, one portion of the rising wall60 as well as the corner portion 65 comes in contact with the inner wallof the prepared hole. Accordingly, the biting property of the screwthread 40 into the prepared hole is improved and the female screw can bemore efficiently formed in the prepared hole. Further, the female screwcan be precisely formed in the prepared hole since the plural notchportions 45 are formed so as to be equally located in thecircumferential direction of the screw thread 40.

The present invention is not limited to the above embodiment modes, butcan be variously modified. For example, in the above first embodimentmode, the outer shape viewed from the axial direction of the screwthread is set to the square shape. However, the outer shape of the screwthread is not limited to the square shape, but may be also set to apolygonal shape of a triangular shape or a pentagonal shape or more.Further, in this embodiment mode, the outer shape of the screw threadfrom the reduced diameter portion 3 b to one portion of the constantdiameter portion 3 a is set to the square shape. However, no range forsetting the outer shape of the screw thread to the square shape islimited to the range shown in this embodiment mode. Similarly, in thesecond embodiment mode, the notch portion 45 is formed in the screwthread from the reduced diameter portion 33 b to one portion of theconstant diameter portion 33 a. However, no range for forming the notchportion 45 is limited to the range shown in this embodiment mode.Further, in the above second embodiment mode, the notch portions 45 areformed every four notch portions so as to be overlapped with each otherwhen they are seen from the bottom face with respect to the screw thread40 of one pitch. However, the number of notch portions 45 is not limitedto four, but can be suitably changed. In addition, the shape of thenotch is not limited to that in this embodiment mode, but may be alsoset to a shape in which the screw is precisely formed in the preparedhole of the attached member.

1. A tapping screw comprising a head portion and a shaft portionextending from said head portion, and forming a female screw in aprepared hole formed in an attached member while the tip portion of saidshaft portion is screwed into said prepared hole; wherein said shaftportion has: a columnar constant diameter portion extending from saidhead portion and having a constant diameter; a reduced diameter portionof a truncated cone shape extending from the tip of said constantdiameter portion in the direction opposed to said head portion, andreduced in diameter as it is advanced in the direction opposed to saidhead portion; and a spiral screw thread continuously formed over theouter circumferential face of said constant diameter portion and theouter circumferential face of said reduced diameter portion; pluralnotch portions are formed in a specific area of said screw thread in thecircumferential direction of said shaft portion; and said notch portionhas a notch face, a rising wall and a corner portion.
 2. The tappingscrew according to claim 1, wherein said notch face has a stem facenotched and formed on the diametrical inside from the top of said screwthread.
 3. The tapping screw according to claim 1, wherein said risingwall is approximately perpendicular to the screw rotating direction. 4.The tapping screw according to claim 1, wherein said corner portion isformed at a diametrical outside end of said rising wall.
 5. The tappingscrew according to claim 2, wherein said stem face has one end connectedin a position moved a predetermined angle a from said corner portion tothe backward side in the rotating direction, and also has a surfacemoved in the diametrical inside direction as it proceeds to the backwardside in the rotating direction, and further has the other end connectedto the end portion of said rising wall on its diametrical inside.
 6. Thetapping screw according to claim 1, wherein said screw thread is left insaid specific area.
 7. The tapping screw according to claim 1, whereinthe top of said screw thread is shaved in said specific area.
 8. Thetapping screw according to claim 2, wherein the screw thread low inheight on said stem face is left in said specific area.
 9. A tappingscrew comprising a head portion and a shaft portion extending from saidhead portion, and forming a female screw in a prepared hole formed in anattached member while the tip portion of said shaft portion is screwedinto said prepared hole; wherein said shaft portion has: a columnarconstant diameter portion extending from said head portion and having aconstant diameter; a reduced diameter portion of a truncated cone shapeextending from the tip of said constant diameter portion in thedirection opposed to said head portion, and reduced in diameter as it isadvanced in the direction opposed to said head portion; and a spiralscrew thread continuously formed over the outer circumferential face ofsaid constant diameter portion and the outer circumferential face ofsaid reduced diameter portion; and the outer shape seen from the axialdirection of said screw thread is set to be non-circular in the specificarea of said screw thread.
 10. The tapping screw according to claim 9,wherein the outer shape of said constant diameter portion seen from theaxial direction of said screw thread is set to be non-circular over apredetermined range in the axial direction from the boundary of saidconstant diameter portion and said reduced diameter portion.
 11. Thetapping screw according to claim 9, wherein the shape of said screwthread seen from its axial direction is a square shape, and the screwthread formed in the outer circumferential portion of said shaft portionand including the screw thread of the square outer shape seen from theaxial direction rises in all forming areas without breaking its topportion.